I don't know if it would help, but the m/n/ng sounds are nasals, and follow the /b/d/g articulation spots. So I suppose if your students could say "mb" (sort of mmmmmmbuh) and then "nd" (sort of nnnnnnnnduh) maybe they could get to "ng" (sort of ngngngngngguh) and then leave off the "guh". If that made sense, congratulations . Pretty hard to show what you mean in print.

Try a word that doesn't have any alveolar sounds in it--"cling" is too hard. Try "king" or better yet, "gong". Having the student pronounce "gong" while keeping the tip of the tongue firmly against the lower teeth. The /ng/ should come out correctly. If the student can't do this on her own, then use a spoon to put downward pressure on the tongue tip while she says "gong".